Conventionally, in an image forming device which uses a powder developer constituted by a toner and a carrier, measurement of developer density, namely, weight mixture ratio of the toner to the carrier (hereinafter, referred to as "toner density"), has been performed, in order to keep density of an image properly.
The measuring technique for the toner density is roughly classified into two methods.
In a first method, permeability of the developer is measured by a magnetic sensor which is provided on a developed agitator in a development apparatus, and the toner density is derived from a measured value (hereinafter, this method is referred to as "magnetic measurement method").
In a second method, the toner density in the developer is measured from quantity of reflected light from the developer which is illuminated through a transparent detection window (hereinafter, this method is referred to as "optical measurement method").
When the magnetic measurement method and the optical measurement method are compared with each other, the optical measurement method has advantages that the toner density can be measured more directly, and the measurement becomes more insensible to variation of the developer caused by variation of an environment and variation of bulk density in the optical measurement method than in the magnetic measurement method. For example, in the optical measurement method, an error in the measurement is not caused even if a condition of humidity varies. On the other hand, the optical measurement method has a drawback that an error in the measurement is caused by adhesion of the developer onto the detection window.
In order to eliminate the drawback of the known optical measurement method, other optical measurement methods (1) and (2) are proposed. In the method (1), the toner density is measured by illuminating the magnetic brush of the developer retained on a rotary sleeve in a development section and the magnetic brush is brought into contact with the transparent detection window so as to clean the detection window. In the method (2), the toner density is measured by illuminating the developer contained in the developer agitator and a magnet is provided on a rotary agitator member of the developer agitator such that the detection window is cleaned by the developer adhering to the magnet.
However, according to the method (1), the sleeve must be rotated in order to measure the toner density. If the sleeve is rotated during non-development period, powder smoke and toner falling from the sleeve occur, and such an undesirable situation may occur that the interior of the development apparatus is contaminated.
Therefore, in a device for forming a multi-color image which is provided with a plurality of development apparatuses containing respectively a developer of a different color, and forming an image by bringing each development apparatus alternatively one by one into contact with a photosensitive drum, the above method (1) should not be adopted, because if the toner density in the development apparatuses which are remote from the photosensitive drum are measured by the above method (1), not only the interiors of the apparatuses are contaminated by the powder smoke and the falling toner, but also the image may be adversely affected. Especially, in the device in which the development apparatuses are arranged one on the other in a row, if the toner falling down from upward adheres to the sleeve of the development apparatus which is developing, or to the photosensitive drum, the image may be adversely affected seriously. Due to these circumstances, it is impossible to adjust the toner density in advance when the development apparatuses are remote from the photosensitive drum so as to develop in no time when each development apparatus faces the photosensitive drum. Further, due to these restrictions, it takes a long period of time to complete a full-color image, resulting in obstacle to realization of high speed device.
On the other hand, in the above method (2), since a flow of the developer, and consequently a level of the surface varies with the rotation of the developer agitator member, this method has drawbacks that amount of the developer does not be stabilized, and correct toner density can not be measured.
Further, if amount or bulk density of the developer, which is in contact with the detection window, varies, a result of measurement varies with it. In other words, there is a drawback that the result may indicate the developer density is improper, even developer density is proper.